Im on mac os. Also - where does Bitcoin-Qt save the main wallet.dat file that it uses?
(Im a fairly new Mac user too so still working out where the app files are etc)
Go to /Library/Application Support/Bitcoin to find bitcoin's data. wallet.dat lives there.
To dump a private key, do thus:
1.) From Menu, click Help > Debug Window
2.) Click on Console
3.) type dumpprivkey [bitcoin address]
I keep hearing really good things about Armoury ... so do you justsend your coins from Bitcoin-qt to yourself in your new Armoury wallet?
T.
Or you could import the private key you just dumped into Armory. I assume Armory has an import key command.